Proton exchange membrane fuel cells, also known under the name of polymer electrolyte membrane fuel cells, (PEMFC) are fuel cells of a type developed for applications in transportation and also for portable applications. The principle of fuel cells was demonstrated experimentally in 1839 by the British electrochemist Sir William Grove. The first fuel cells of PEMFC type were developed in the United States from the 1960s by General Electric for space applications. Currently, cells of this type, designed to operate at intermediate temperatures (40-120° C.), have been developed internationally by the motor vehicle and portable electronics industries. However, despite undeniable environmental advantages and high energy efficiencies, fuel cells are only just beginning to compete with internal combustion engines because of costs which are still high (starting materials, lifetimes).
The core of a fuel cell of PEMFC type is composed of a polymer electrolyte membrane, of electrodes (anode and cathode, generally in the form of thin layers of platinum) and of bipolar plates used for gas diffusion.
Fuel cells operating with proton exchange polymer electrolyte membranes make it possible to convert the chemical energy of gases (H2/O2) into electrical energy with high energy efficiencies and without any discharge of pollutant, according to the following equations: Reaction at the cathode (site of the reduction of the oxygen):½O2+2H++2e−→H2O
Reaction at the anode (site of the oxidation of the hydrogen):H2→2H++2e−
The two electrodes being separated by the electrolyte (membrane), the fuel to be oxidized (hydrogen) is conveyed to the anode and the cathode is fed with oxygen (or more simply with air, which may or may not be enriched in oxygen). The dihydrogen reacts at the anode and releases two electrons (oxidation) which feed an external electrical circuit connecting the anode and the cathode. Cathodic reduction of the oxygen takes place at the cathode. The reactants are in principle introduced continuously into the device and the electromotive force of the cell is equal to the difference in the electrode potentials. Thus, a universally known overall reaction is obtained:H2+½O2→H2O
Water is thus produced by the normal operation of the cell and has to be discharged to the outside of the membrane. The management of the water is crucial for the performance of the cell; care should be taken that the amount of water remains continually at an optimum level guaranteeing that the cell operates well. In particular, an excess of water results in excessive swelling of the membrane and in blocking of the distribution channels or electrodes and has a negative effect on the access of the gases to the catalytic sites, whereas an inadequate amount of water results in draining of the membrane, which is harmful to the conductivity thereof and to the efficiency of the cell.
The role of the membrane is thus to provide for the transportation of the protons (H+) from the anode to the cathode and to thus make possible the electrochemical reaction. However, the membrane must not conduct the electrons, which would create a short circuit in the fuel cell. The membrane must be resistant to the reducing environment at the anode and, at the same time, to an oxidizing environment at the cathode but must also prevent the mixing of the hydrogen present at the anode with the oxygen present at the cathode.
One of the first proton-transporting polymers which was used for the production of such membranes, and which today remains the reference in this field, is Nafion®, a perfluorosulfonic polymer developed and perfected in 1968 by the American firm Du Pont de Nemours. Historically, the Gemini space programs of NASA in the 1960s used fuel cells comprising membranes of polystyrenesulfonate type but they were very quickly supplanted by the Nafion® membranes, which made it possible to improve the performance of the PEMFCs. Chemically, it is an organic polymer composed of a flexible fluorocarbon chain on which ionic groups are randomly distributed (Mauritz K. A. et al., Chem. Rev., 2004, 104, 4535-4585). On the principle of Nafion®, there also exist other commercial perfluorosulfonic polymers, such as those sold under the trade names Aciplex® (Asahi Chemical Company, Japan) or Flemion® (Asahi Glass Company, Japan).
The membranes manufactured from these polymers are by nature very stable chemically, thermally and mechanically (flexibility). They exhibit good electrochemical properties with a high conductivity, of the order of 0.1 S·cm−1 at ambient temperature and 100% relative humidity (according to the data of the manufacturer for Nafion®). However, these membranes have to operate at a temperature of less than 90° C. and always have to remain saturated with water in order to make possible effective movement of the H+ ions. This is because the conduction of the protons takes place mainly by a mechanism of Grotthus type, that is to say by protons hopping along the ionic and hydrophilic conduction pathways (Mauritz K. A. et al., 2004, abovementioned). Furthermore, the synthesis of these membranes is lengthy and difficult, indeed even dangerous due to the use of fluorine, which partly accounts for their very high cost price. Neither are they entirely satisfactory as regards the problems related to the management of the water and to the changes in temperature. This is because, when the system is subjected to numerous variations in the level of moisture, successive cycles of swelling and of restructuring of the membrane are noted as appearing, resulting in significant fatigue. Furthermore, Nafion® is a polymer which very naturally undergoes a glass transition (Tg=120° C.), which contributes to its accelerated aging and to the appearance of structural reorganizations and of mechanical weaknesses (splits), thus limiting its lifetime.
Other types of alternative polymers which can be used in the preparation of electrolyte membranes have also already been proposed. They are in particular sulfonated or doped heat-stable polymers (polybenzimidazoles, polyarylethersulfones, sulfonated polyaryletherketones, and the like). These polymers result in membranes also exhibiting some disadvantages, in particular in terms of conductivity (performance), of lifetime and of management of the water.
Patent application US 2005/0164063 describes the synthesis of various solid compounds and electrolytes obtained from precursors based on silsesquioxane in which a siloxane functional group is bonded to a phenylsulfonate group via a divalent group devoid of a urea functional group. Such structures, in which the divalent group connecting the siloxane functional group to the phenylsulfonate is an alkyl or aryl radical, have the disadvantage of exhibiting low conductivities (Electrochimica Acta, 2003, 48, 2181-2186).
This is why, in order to overcome the respective weaknesses of each of these systems, numerous studies on modification by incorporation of inorganic phases have been carried out in recent years, which has resulted in an overall improvement in the properties of PEMFCs. This is reflected in particular in the management of the water and in the behavior of the materials at high temperature (dehydration) and their long-term stability. These concepts have spread with the appearance of hybrid membranes, which have also made it possible to demonstrate the importance of the presence of a continuous inorganic network within the conducting electrolyte.
Rhodium-based monomeric complexes obtained by reaction of (p-aminophenyl)diphenylphosphine with (3-isocyanato-propyl)triethoxysilane, exhibiting improved catalytic properties, are also known and used for sol-gel polymerizations (J. Organomet. Chem., 2002, 641, 165-172).
Films prepared by reaction of an alkoxysilane with 4-[(4″-aminophenyl)sulfonyl]-4′-[N,N′-bis(2-hydroxy-ethyl)amino]azobenzene, by sol-gel polymerization, are also described in Chem. Mater., 1998, 10, 1642-1646, for their applications in the field of optics.
Unfortunately, at the present time, no membrane, whatever its nature, fully meets the stringent requirements of the manufacturers and users of PEMFCs. Although many operational technical devices using these electrochemical systems have appeared on the market, such as, for example, the GENEPAC fuel cell, which results from a partnership between PSA Peugeot Citroen and the Commissariat à l'Energie Atomique [French Atomic Energy Commission] and which has a power which can range up to 80 kW, there still remain technological blockades to be raised.
In terms first of all of management of the water: as was seen above, it is essential to manage the water produced during the operation of the cell and its influence on the properties of the electrolyte membrane (in particular the conductivity). This need to control and manage as best as possible the transportation of water which takes place in a PEMFC (entries, exits, generation and back-diffusion between cathode and anode) remains a very major constraint which encourages the production of electrolytes less dependent on the relative humidity.
In terms of operating temperature also: fuel cells of Nafion® type can only operate at maximum temperatures of 90° C. For higher temperatures, the membranes may no longer provide suitable conductivity of the protons because of their inability to retain the water. Their efficiency decreases as a function of the fall in the relative humidity related to the rise in the temperature. In point of fact, the application of fuel cells in transportation vehicles requires the use of membranes which can operate satisfactorily at temperatures of greater than 90° C., in particular at temperatures of between 120 and 150° C. Membranes of this type do not currently exist on the market.
In terms finally of manufacturing cost: in order to make it possible to develop this technology on a large scale and to render these power generators, destined for a bright future, widespread, the problem remains of the cost of manufacture of the electrolyte membrane as such but also the cost of manufacture of the fuel cell core (MEA) related to the use of platinum as catalyst.